In recent years, a carbon nanotube or a new carbon material is prospective particularly in the application as an emitter material such as for a field emission cold cathode. A CNT has the shape of a hollow cylinder of a graphene sheet which has regularly arranged carbon atoms and which is formed in a tubular shape, and is a very high aspect ratio microscopic material which has an outer diameter of the order of nanometers (nm) and a length of 0.5 to tens of micrometers. The CNT having such a shape readily provides an electric field concentration at its tip portion, and can be expected to provide a high emission current density. Furthermore, the CNT has highly stable chemical and physical characteristics, and is therefore expected to be stable against such as adsorption of residual gases in a working vacuum or ion bombardments.
There exist two types of CNTs: a single-layer nanotube and a multi-layer nanotube. The single-layer nanotube is a tube having the thickness of a monoatomic layer and one sheet of graphene (hexagonal carbon mesh plane of a monoatomic layer) closed in a cylindrical shape, being about 2 nm in diameter. The multi-layer nanotube has a stack of multiple layers of cylindrical graphenes, being 5 to 50 nm in its outer diameter and 3 to 10 nm in diameter of the central cavity. The single-layer nanotube, which is frequently used as an emitter, can be produced by arch discharge with carbon bars employed as the electrodes. This production method is described in a literature such as Nature, Vol. 354 (1991), pp56–58, in which a description can be found which recites that arch discharge is performed in a helium or argon gas environment at 66,500 Pa (500 Torr) using carbon bar electrodes doped with iron, cobalt, or nickel as a catalytic metal.
On the other hand, a transfer method for depositing CNTs in the shape of film is described, for example, in Science, Vol. 268 (1995), P. 845, and in Science, Vol. 270 (1995), P. 1179. In this transfer method, a CNT slurry having CNTs scattered in the solution is filtered with a ceramic filter having a pore size of 0.2 μm, and then the reverse side of a film of CNTs remaining on the filter is pressed onto a substrate, only the filter being stripped away thereafter. This allows a thin film containing CNTs to be formed on the substrate.
To apply the CNT film formed as described above to a display, the CNT film is used for the cathode (emitter) serving as an electron source. In a diode structure with an anode electrode and a phosphor disposed in close proximity thereto, as described in Appl. Phys. Letters, Volume 72, p. 2912, 1998, for example, a voltage of 300V is applied between the anode electrode and the emitter which oppose each other, and the electrons emitted from the emitter hit and excite the phosphor on the anode electrode side to emit light, thereby allowing characters or the like to appear on the display.
By way of example, a FED of a triode structure is shown in FIG. 28A. In the triode structure, an emitter 112b using CNTs is employed as a field emission cold cathode, with a gate electrode 125 disposed between the emitter 112b and an anode electrode 124. A conductive substrate or a conductive layer 111 is formed on a glass substrate 110, a CNT film 112 is deposited on the conductive layer 111, and the gate electrode 125 is formed on the CNT film 112 via an insulating film 23.
Furthermore, a gate opening 117 penetrating the gate electrode 125 and the insulating film 123 allows a portion of the CNT film 112 to be exposed, thereby forming the emitter 112b. The anode electrode 124 is disposed above and spaced a predetermined distance from the glass substrate 110 containing such as the CNT film 112 and the gate electrode 125, with the space therebetween being maintained under vacuum. In such a triode structure, a negative potential is applied to the CNT film 112 while positive potentials are applied to the anode electrode 124 and the gate electrode 125, respectively, thereby making it possible to emit electrons from the emitter 112b exposed within the gate opening 117 toward the anode electrode 124.
To fabricate a flat image display device such as FEDs using the aforementioned triode structure, an insulating film is formed on a CNT film and an opening is then formed in the insulating film using an etching solution or an etching gas or the like, wherein those CNTs that stand upright near the surface of the CNT film may disappear due to the influence of the etching solution or the etching gas, thereby impairing excellent characteristics of electric field concentration.
A CNT film fabricated according to a prior art fabrication method is shown in FIG. 28B. In this fabrication method, a liquid mixture having CNTs 112a scattered in a binder solution is coated onto the conductive layer 111 on the surface of the substrate 110, and the CNT film 112 is formed while the adhesion between the substrate 110 side and the CNTs 112a is being enhanced. With this method, for example, most CNTs 112a on the CNT film surface lie down toward the substrate surface due to the viscosity and the surface tension of the binder solution or are buried in the binder, thereby impairing their upright states and making it extremely difficult to implement uniform emission characteristics at low voltages.
The binder is often composed mainly of an insulating material such as resist, water glass, and acrylic resin. When the surface of the CNT film 112 is coated with this insulating material, the surface barrier of electrons is substantially increased upon emission of the electrons, thereby significantly reducing the emission efficiency. This may enhance the adhesive strength between the substrate 110 and the CNT film 112; however, an emitter having CNTs 112a not aligned upright cannot make full use of the advantage of being provided with the CNT film.